1. Field of the Invention
The present invention relates to a process for producing polymeric gasolines and jet fuel from saturated C.sub.4 hydrocarbon fractions.
2. Description of the Background
The exploitation of C.sub.4 fractions obtained as a byproduct from FCC (catalytic cracking on fluidized bed=fluid catalytic cracking) and steam cracking is becoming an increasingly serious problem in petroleum and petrochemical industries. In particular, interesting is the exploitation of the paraffinic fraction, and, mainly, of n-butane, remaining as a residue after the treatments for recovery and exploitation of olefins (e.g., separation of butadiene, esterification of isobutene and methyl-t-butyl-ether, oligomerization and alkylation of isobutane and n-butenes). In particular, in Italian Patent AppLication No. 21,157 A/90, n-butane is used in a process for obtaining isobutene-containing olefinic fractions, which constitutes a valuable intermediate which is useable in such chemical reactions as polymerizations and alkylations, and in isoprene production. The catalyst used for this transformation is based on platinum supported on alumina whose surface is coated with silica, possibly in mixture with a solid acidic catalyst selected from alumina surface coated with silica or Boralite B.
A process has now been found by which it is possible to use C.sub.4 paraffinic fractions, essentially constituted by n-butane, as a starting material in conversion to valuable hydrocarbonaceous fractions suitable as engine fuels.
Several processes are known for preparing hydrocarbon fractions which are useful as fuels, by starting from light C.sub.3 -C.sub.4 olefins in the presence of solid acidic catalysts.
In U.S. Pat. No. 3,960,978; EP-31,675 and U.S. Pat. No. 4,150,062 use is claimed of zeolites of the ZSM-5 type, in order to produce gasolines by olefin oligomerization. Unfortunately, the so obtained gasolines thus prepared also contain a fraction of aromatic hydrocarbons, mainly benzene. Such substances are obviously undesirable, because of their harmfulness to man.
In U.S. Pat. No. 4,227,992; U.S. Pat. No. 4,456,779 and U.S. Pat. No. 4,720,600, processes for light olefin oligomerization, catalyzed by zeolites of the ZSM-5 type, are disclosed, which make it possible to obtain products which contain a useful hydrocarbon fraction for use as jet and diesel fuel. However, these processes require that decidedly significantly high temperatures, i.e., of at least 250.degree. C., are used.
At present, from an industrial viewpoint, the most widely used catalyst for producing polymeric gasoline is supported phosphoric acid (J. F. McMahon et al., "Polymerization Of Olefins As A Refinery Process", Adv Pet Chem., Volume VII, 1963, pages 285-321). This catalyst does not require as high reaction temperatures as those required by the processes with zeolite catalyst, but is affected by a number of other drawbacks: in fact, it is corrosive, when exhausted is not reclaimable, and disposing of it causes environmental problems. Furthermore, the polymeric fractions obtained are nearly exclusively light fractions, and jet and diesel fuels are not produced.
In Patent Application EP-340,868 amorphous silica and alumina gel, as shown by X-ray analysis, has a molar ratio of silica:alumina which is within the range of from 30:1 to 500:1, a surface area within the range of from 500 to 1000 m.sup.2 /g, and a pore diameter substantially within the range of from 1 to 3 nm. Such a catalyst can be advantageously used in the dimerization of linear C.sub.4 -C.sub.15 olefins, in isobutene dimerization and propylene oligomerization.
Italian Patent Application 91 A 003 276, discloses the preparation of a silica-and-alumina-gel-based extruded catalyst which is very effective in propylene oligomerization.